The present invention generally concerns spinal fixation systems, such as systems for use in the fixation of the spine for the treatment of various spinal deformities. Specifically, the present invention contemplates a lateral connector assembly for interconnecting an elongated member, such as a plate or a rod, with a bone engaging fastener, such as a hook, screw or bolt.
The treatment of spinal deformities and injuries has evolved significantly over the last 30 years. Spinal deformities, such as scoliosis and kyphosis, as well as fractures, spondylolisthesis, and other medical indications have been treated using a system of relatively rigid elongated members spanning the vertebral column. In one type of system, the elongated members constitute a plate that has a number of openings or slots through which bone bolts or bone screws extend. The bone engaging fasteners are threaded into different aspects of the vertebra to achieve fixation of the elongated plate. One system of this type is the DYNA-LOK.RTM. Spinal System provided by Sofamor Danek Group, Inc. of Memphis, Tenn.
In another approach, the system includes a generally rigid spinal rod sized to span along the length of the vertebral column. In this system, a variety of bone engaging fasteners are connected to the spinal rod by way of a clamp. In some systems, such as the CD.TM., CD HORIZON.TM. and LIBERTY.TM. spine systems marketed by Sofamor Danek, the fixation elements are threaded directly onto the spinal rod and held in place by a system of set screws. In this approach, the bone engaging fastener is situated directly in line with or beneath the spinal rod. The LIBERTY.TM. System also includes the STAR.TM. connector which is threaded onto the rod and provides for lateral connection of the same bone engaging fasteners.
In another type of rod-based spinal system, the bone engaging fasteners are connected laterally to the spinal rod. Two such systems are the TSRH.RTM. and the GDLH.RTM. spinal systems provided by Sofamor Danek. In these two systems, arrangements of eyebolts or T-bolts are threaded onto the rod and provide an interface to the bone engaging fastener, such as a hook or a screw. The TSRH.RTM. Spinal System includes bone screws, with one type having a central post that is engaged directly to the spinal rod, and another type providing variable angle capability, such as the TSRH.RTM. variable angle bone screw as shown and described in U.S. Pat. No. 5,261,909, to Sutterlin et al. As disclosed in the '909 Patent, which disclosure is incorporated herein by reference, the variable angle bone screw can achieve variable angular positions in a plane adjacent and perpendicular to the radius of the spinal rod. The TSRH.RTM. variable angle screw thus provides an additional degree of freedom for attaching the bone engaging fastener to the spinal rod over the fastener having a central post.
It is of course known in the field of spinal instrumentation that the fixation system must adapt to the spinal anatomy, both the original anatomy and the desired corrected anatomy. Movement or manipulation of the components of the system in different degrees of freedom has become essential to achieving proper correction and fixation. Consequently, the TSRH.RTM. Spinal System is provided with T-bolts for engaging the variable angle screws that provide an additional degree of freedom by laterally displacing the screw from the rod at predetermined distances. Lateral offset plates are also included which can be moved to adjustable lateral distances from the spinal rod to connect to the bone engaging fastener.
In the other type of rod-based system, as illustrated by the GDLH.RTM. posterior spinal system, bone bolts can be provided for fixation to the vertebrae. The bone bolts include a lower portion threaded for engagement in the vertebral bone, and an upper machine threaded portion. The GDLH.RTM. System includes various rod/bolt connectors for attaching the bolt to a spinal rod. In one type of GDLH.RTM. rod/bolt connector, the connector includes a single hole through which the machine threaded portion of the bone engaging fastener extends. A nut is threaded onto the machine threads to clamp the bone bolt to the connector. In this type of rod/bolt connector, the bone bolt is held at a fixed lateral position relative to the spinal rod. In another type of GDLH.RTM. rod/bolt connector, the connector includes an elongated opening or slot which allows relative lateral movement of the bone bolt with respect to both the connector and the spinal rod. With this GDLH.RTM. rod/bolt connector, the bone bolt can be engaged to the rod at variable lateral distances from the rod. Again, this feature allows the surgeon to accommodate variations in the spinal anatomy with respect to the placement of the spinal rod.
In the implantation of any spinal instrumentation, one goal of the surgeon is to minimize the intrusion into the patient, whether by the amount of implants that must be used or by the length of time required to fix the implants within the patient. In the early days of spinal fixation, the surgeon was limited in the locations and manner in which the bone engaging fasteners could be fixed to the vertebrae, and ultimately attached to an elongated member, such as a plate or a rod. As the spinal fixation systems became more elegant, the surgeon was been provided with additional degrees of freedom for movement and positioning of the bone engaging fasteners and the elongated members, either individually or with respect to each other. The use of lateral connectors between the bone engaging fastener and a spinal rod, for instance, has provided an additional degree for freedom to the surgeon. Adding the elongated slot for engagement to a bone bolt, such as in the GDLH.RTM. Spinal System, or providing T-bolts of varying lengths to engage the variable angle bone screw of the TSRH.RTM. Spinal System, adds a further degree of freedom. With these components providing lateral variability of the connection between the fastener and the rod, the surgeon need not bend the rod laterally to meet the position of the bone engaging fastener. For example, in some indications, a bone bolt is threaded into the pedicle of the vertebra. During the correction process, the spinal rod may not be directly aligned over or adjacent to the top of the bone bolt. In prior less sophisticated systems, the surgeon was required to bend the rod laterally so that the rod could be positioned directly adjacent the bone engaging fastener for connection between the two components. The addition of the lateral variability provided by the GDLH.RTM. and TSRH.RTM. components discussed above eliminates this bending requirement.
While this lateral adjustment feature has proven to very beneficial to spinal surgeons, additional degrees of freedom can still greatly improve the surgeon's task in quickly and efficiently instrumenting a patient's spine. The starting point of any spinal surgery is the spine itself, and more specifically the position and orientation of each of the vertebrae to be instrumented. Each vertebra has only a few "engagement points" onto which can be affixed a bone engaging fastener, such as a hook, screw or bolt. If that engagement point of the vertebra does not match the location of the connector for fixing the fastener to the elongated member, the surgeon must either manipulate the vertebra, or manipulate and/or bend the elongated member. In order to avoid this mechanical manipulation, it is desirable to provide the surgeon with additional degrees of freedom to more easily accommodate all possible orientations of a bone engaging fastener when it is engaged to a vertebra.
Consequently, there remains a need for providing a means for connecting a bone engaging fastener with an elongated member not only at variable lateral positions, as provided in some prior systems, but also at additional degrees of freedom relative to the rod. This need specifically encompasses variable angular orientations between the bone engaging fastener and the elongated member to which the fastener is to be connected.